Our previous studies revealed that Wilms' tumor 1 (WT-1) protein was highly expressed in breast myoepithelial (ME) and endothelial cells. As the human breast tissue is rich in ME cells and blood vessels, our current study intended to assess whether WT-1 immunohistochemistry may have dual usages in evaluation of the ME cells and micro-vessel density. Consecutive sections were prepared from breast tumors with co-existing normal, hyperplastic, and neoplastic components. Consecutive sections were immunostained for WT-1 and a panel of ME and endothelial cell markers. From each case, 4-5 randomly selected duct clusters were photographed, and the percentages of positive cells for these molecules were compared. Similar to ME cell marker CD10 and smooth muscle actin (SMA), WT-1 expression was preferentially seen in ME cells, and over 90% of WT-1 positive ME cells were immunoreactive to CD10 and SMA. Distinct WT-1 expression was also seen in endothelial cells, and over 90% of WT-1 positive endothelial cells were positive for blood vessel specific markers. With tumor progression, the percentage and intensity of WT-1 positivity decreased in ME cells, whereas increased in endothelial cells. These finding suggest that WT-1 immunohistochemistry may be used to assess both the ME cells and micro-vessel density.